The vast majority of all heat transfer applications are primarily concerned with heat transfer between two fluids, which necessarily involves an interfacial solid surface. This invention, however, relates to a distinctly different class of applications in which the heat transfer design addresses only that portion of the system involving heat transfer between an objective surface and a working fluid.
Examples of low thermal flux surface-fluid transfer applications can be found in the following: (1) solar energy, where tubes are bonded to a surface that is exposed to the sun, such as that by Bowen in U.S. Pat. No. 4,150,657; (2) electronics, where finned surfaces are commonly employed to cool a surface that has been conduction heated by electronic devices, such as described by Friedman et al in U.S. Pat. No. 4,478,277; and (3) manufacturing processes, such as described by Hedin in U.S. Pat. No. 4,003,687 and Diener et al in U.S. Pat. No. 4,335,870. Wittel, U.S. Pat. No. 4,583,583, uses spring-like inserts in 6 mm I.D. fluorocarbon tubing for crimp resistance in forming serpentine fluid paths between grooved metal blocks for moderately low flux applications in fuel cells. Andres et al, U.S. Pat. No. 4,550,774, use heat pipe condenser plates to distribute heat to vehicle surfaces at moderately low flux. Germann, U.S. Pat. No. 4,266,603, discloses a method of forming tubes for fluid passage from jaws on an extruded surface. Darling et al, U.S. Pat. No. 4,294,199, cover the surface of a magnetohydrodynamic (MHD) diffuser with parallel tubes for moderate flux at high temperatures. Typical heat fluxes in the above examples are about 1 kW/m.sup.2 in solar energy, and 10-50 kW/m.sup.2 in most other applications, but fluxes below 100 W/m.sup.2 are the object of certain solar heating applications, such as Platell, U.S. Pat. No. 4,186,795.
Electronic device cooling applications have led to the development of compact, high thermal flux, surface-fluid exchangers. Heide et al, U.S. Pat. No. 4,161,213 describe a cooling capsule for a thyristor that utilizes a drilled, metallic core with electron-beam welded header plates completing the fluid-tight serpentine path. Ruger, U.S. Pat. No. 4,161,980, encapsulates a bifilar wound (stainless) tube in an aluminum capsule for the same purpose. Iversen, U.S. Pat. No. 4,712,609, utilizes transverse pressure gradients for vortex induced turbulence and nucleate boiling enhancement in a micro-channel extrusion with special surface preparation to achieve thermal fluxes up to 13 MW/m.sup.2 with bi-phasic fluids.
Several additional applications have developed for low mass, high thermal flux, surface coolers. Solar cells as used in space power applications require low mass, low thermal gradient, back side cooling at fluxes up to 0.1 MW/m.sup.2 in high ratio concentrators. Stultz, U.S. Pat. No. 4,397,303, describes a multilayer, multi-channel extrusion for use with a solar concentrator. Little, in U.S. Pat. Nos. 4,386,505, 4,392,362, and 4,489,570, discloses the use of multilayer glass laminate structures with micron sized channels, lithographically etched, for high flux, low mass, heat transfer in Joule-Thompson refrigerators and in surface cooling applications, especially for IR detectors. Little's laminar flow devices are relatively fragile, expensive, and not suitable for high temperature applications.
Leading edges of hypersonic aircraft are subject to friction heating in excess of 0.5 MW/m.sup.2. Rocket nozzles and diffusers require surface cooling of 0.2 to 15 MW/m.sup.2. Fuel injector struts in scram jets require cooling at 5 to 15 MW/m.sup.2. Niino et al, U.S. Pat. No. 4,703,620, use a porous wall to achieve such ultra high fluxes with sacrificial perspiration of the working fluid through the wall. In all of the above aerospace applications, exchanger mass is crucial.
The appropriate figure-of-merit in these aerospace applications is specific conductance, measured in W/kgK, where the total system mass and total temperature difference T.sub..delta. are considered along with total power transfer. (The thermal siphon literature has often confused the central issue by separate analyses of power-distance products, surface conductance in terms of W/m.sup.2 K, film transfer coefficients, etc.) Prior art surface-to-helium pumped loops have achieved about 10-50 W/kgK, and very short (3 cm) liquid lithium heat pipes have achieved about 800 W/kgK, but heat pipes are effective only at short distances--they degrade rapidly at distances above 50 cm. The present invention is capable of achieving 300-1500 W/kgK with helium gas pumped loops, which are nearly independent of source to sink transfer distance.
The first wall of a controlled thermonuclear fusion (either inertial or magnetic confinement) reactor chamber may experience pulse heat fluxes in excess of 50 MW/m.sup.2 and may require average cooling rates in excess of 2 MW/m.sup.2. It is desirable to provide such first-wall cooling in a fusion reactor by means of an exchanger that has minimal absorption of fast-spectrum neutrons and--in the case of magnetic confinement --minimal MHD interactions, as discussed by Werner et al, U.S. Pat. No. 4,394,344.
In these applications surface conductance is the more appropriate figure-of-merit, providing a pumped loop is used that avoids distance transfer problems, neutron absorption, and MHD interactions. Prior art surface conductances for pumped helium loops have not generally exceeded 1 kW/m.sup.2 K. (Very short liquid lithium heat pipes have achieved over 50 kW/m.sup.2 K.) The present invention permits over 10 kW/m.sup.2 K with pumped helium loops. Prior art laminar flow, single phase, surface-helium exchangers have generally not exceeded 0.1 MW/m.sup.2, except for Little's devices, which may achieve 0.5 MW/m.sup.2. The present invention, a laminar flow, single phase exchanger, is capable of achieving continuous surface fluxes over 10 MW/m.sup.2 with helium gas, and somewhat higher fluxes are possible with hydrogen gas. Moreover, the extremely low mass of the present invention makes it advantageous even for certain low flux applications such as low temperature space radiators at surface fluxes below 1 kW/m.sup.2. This last example is one of the rare variations in which positive heat transfer occurs from the fluid to the surface rather than vice versa.
Prior art high flux surface-fluid exchange has been obtained only by incorporating one or more of the following options: (1) high velocity, highly turbulent fluid flow, and, hence, high pumping power losses; (2) high conductivity liquids such as molten salts or metals, especially lithium and alloys of sodium and potassium, with their attendant materials handling problems; (3) liquid-gas phase change, especially in thermal siphons, which are effective only over a narrow range of thermal, inertial, and gravitational conditions; (4) lithographic laminate structures, which are fragile, expensive, and unsuitable for high temperatures.
Little's devices may have advantages in certain cryogenic applications, and Iversen's designs, Ruger's designs, etc. may be preferable for very small device cooling applications. However, the instant invention has substantial advantages over the prior art in nearly all other high flux, low mass, low neutron absorption, and low MHD applications.
It is usually highly desirable to obtain the requisite heat transfer with minimum practical temperature difference T.sub..delta. between the fluid and the objective surface for one or both of the following reasons: (1) to permit a heat engine to operate between the heat source and the heat sink at the highest possible efficiency; and (2) to improve the strength and service lifetime of the surface being cooled. A noteworthy advantage of the present invention is that it is well suited to utilizing a single phase working gas with low T.sub..delta., thereby greatly facilitating the use of high efficiency heat engines without intermediary exchangers between primary and secondary loops, especially in closed Brayton cycles.